Conventionally, in a tunnel FET (TFET) having a germanium (Ge) channel, source, channel and drain are formed by means of a pin junction formed by ion implantation. A current drive ability is determined depending on band-to-band tunneling (BTBT) by a source junction. For this reason, in order to improve a drive current, a high-density and steep junction is formed; in this way, a tunnel barrier must be thinned to a thickness of 1 to 3 nm. However, the foregoing junction is formed by using ion implantation and activation annealing; for this reason, there is a limit to the control of junction steepness resulting from ion implantation conditions and impurity diffusion.
The following method has been proposed as a method of realizing a steeper impurity profile instead of using ion implantation. According to the foregoing method, silicon (Si) doped with B and As is epitaxially grown to form a source/drain. However, according to this method, impurity diffuses in a channel; as a result, a steep junction is not formed. Therefore, threshold voltage variations inevitably occur due to un-uniformity of impurity distribution after a junction is formed.
On the other hand, a device for achieving the purpose of low energy consumption is required to reduce a leakage current. An off-leakage current is determined depending on BTBT by a drain-junction. For this reason, a low-density and gradual junction of channel/drain is formed, and thereby, there is a need to thicken a tunnel barrier. However, according to the foregoing method, the following issue inevitably arises; namely, a parasitic resistance increases due to low density of a drain.
Moreover, the following device for achieving the purpose described below has been proposed. Specifically, a source is formed by using Ge having a low band gap (0.69 eV), and a channel and a drain are formed by using Si having a high band gap (1.12 eV). In this way, a tunnel barrier of a source terminal is formed thin while a tunnel barrier of a drain terminal is formed thick. However, in the foregoing device, it is not found to solve a problem related to the control of an interface steepness of a junction formed by impurity diffusion and to threshold voltage variations resulting from non-uniform impurity distribution.